Seiren

Siren (サイレン, Sairen) is a "Marine Warfare Angeloid, Type Eta" (水中戦闘用エンジェロイドタイプη（イータ）) built by the Master of Synapse.

Appearance
She strongly resembles the mermaids of legend with her fin. Siren's hands also resemble fins. Her wings are floating around her.

Plot
Siren was impaled in the back by one of Chaos' wings before being devoured and absorbed by her just after she was ordered by the Master of Synapse to kill Tomoki Sakurai and Ikaros.

Master of Synapse
The Man of Synapse is her master and also the person that created her.

Trivia

 * Siren's appearance is based on the Sirens of Greek mythology.